1. Related Applications
The subject matter of the present application is related to the subject matter of the applications "Dynamic Device Independent Image Correction Method and Apparatus" (attorney file no. 100219) U.S. patent application Ser. No. 09/083,203 filed May 22, 1998, and "Device Independent Color Controller and Method" (attorney file no. 100214) U.S. patent application Ser. No. 09/083,202 filed May 22, 1998 filed by the assignee, which are incorporated herein by reference.
2. Field of Invention
The present invention relates to a method and apparatus for automatically mixing colorants to achieve a desired target color, and in particular, to such a method and apparatus configured for use in an ink marking device.
3. Description of Related Art
Known methods and apparatus for mixing colorants to achieve a desired target color in pigment-based and dye-based applications involve human interaction, namely determining if the mixed colorants match the target color by visual inspection. For example, an experienced attendant of a paint mixing machine in a home improvement store will begin with a base color and add one or more other colors to achieve a customer's desired color. The attendant may refer to a predetermined paint mixing guide to determine which colors and how much of each color to add to the base color before beginning the mixing process. Alternatively, the attendant may access color formulations stored in a memory of a computer or similar device. After the paint is thoroughly mixed, the attendant will visually inspect the resulting color and confirm it matches the desired color.
More human interaction is required in the case of colors that have faded or otherwise been transformed, e.g., due to environmental exposure such as solar radiation or a saltwater environment. In these cases, although the identity and proportions of colorants comprising the original color designation may be known, the target color that must be matched is actually a variation (usually a lighter shade) of the original color. In general, achieving the target color in these cases requires repeated mixing and visual inspection steps, because there is no guide that can be consulted.
Other areas in which color mixing is carried out on a trial and error basis with visual inspection include offset printing, wallpapers, fabric coloring and automobile painting, among others. In offset printing, a printer seeking to match a special color will rely on experience and possibly some reference to basic colors in mixing various standard ink colors to match the special color.
Consumers now demand the flexibility to print and copy in colors not previously available in the normal color palette of conventional ink marking devices (e.g., copiers, printers, etc.). In conventional ink marking devices, color printing is typically carried out with four colors (cyan, magenta, yellow and black). Other colors are printed by applying these four colors in a halftone pattern. Thus, no mixing of the four colorants occurs in the conventional ink marking devices. This method is called process color. Instead, two or more of the colorants are printed in adjacent pixels. The eye blends the adjacent pixels, and the sensed color represents an averaging of the individual pixel colors.
The palette of conventional ink marking devices is limited, and many special colors cannot be reproduced. Accordingly, it would be desirable to provide a way to mix a greater range of colors in an ink marking device automatically. The resulting ink, made from a mixture of two or more color inks, is called a spot color or a custom color. The color gamut space obtained in this way is larger by a factor of two or more than the one obtained by process color. Such spot colors are typically printed as solids, rather than halftones. The printed solid spot colors are more spatially uniform than the overlapping halftones which provide the process color approximations to the customer-selected color.
To mix colors automatically, i.e., without the human interaction through visual inspection, the target color and the mixed color must be sensed so that they can be compared. Typical color sensing devices include calorimeters and spectrophotometers. Colorimeters are simpler devices that sense, e.g., the L*, a* and b* characteristics of a particular color. Spectrophotometers are devices that measure light in terms of its intensity at each wavelength. The received light is decomposed into a spectrum, and particular wavelengths are recorded by corresponding photodiodes, photomultiplier arrays, etc.
Over time, a mixed color that originally matched the target color may drift such that a perceptible difference becomes apparent between the mixed color and the target color. Such a difference may arise for various reasons, including inconsistent mixing of the mixed color, changing environmental conditions, varying absorption properties of the media on which the color is being applied, etc. Thus, to match the target color accurately over time, the mixed color must be monitored and, if necessary, adjusted slightly, to minimize any perceptible difference.